An insight into the severe slugging characteristics in a catenary flexible riser

This paper reports the experimental results of the severe slugging (SS) flow characteristics in a downward inclined pipeline-catenary flexible riser system. The non-intrusive optical measurement with high-speed cameras was employed to capture the evolution of liquid slugs and the gas-liquid interface. Five SS patterns are identified in the considered flow velocity range. There are two kinds of severe slugging I (SSI-1 and SSI-2) with the essential difference in the occurrence of the fast blockage stage before the slug formation (SF) stage. The severe slugging II (SSII) is characterized with the longest liquid slug less than a riser length and the absence of slug production stage. The liquid slug is further shortened in severe slugging III (SSIII), exhibiting local liquid fallback and accumulation of multiple slugs in the riser. The severe slugging transition (SST) occurs due to the switching between the SSI-1 and SSII. The flow regime partition is plotted in the v(sl) (liquid superficial velocity)-v(sg) (gas superficial velocity) diagram, presenting alteration as the inclination angle of upstream pipeline varies. Generally, the SSI is easier to form at a higher inclination angle. The intermittent occurrence of hydrodynamic slug in the upstream pipeline in the SF stage contributes to the appearance of SSI-2 to SST. The SS cycle has approximately a negative exponent relation with v(sg), while the riser base pressure changes exponentially with the increase in v(sl).